Polyester films are satisfactory in almost all the properties required as the base film for photography and photoengraving such as transparency and hazelessness; tensile strength; resistance to splitting, torsion, curling, etc.; dimensional stability; resistance to photographic emulsion, water, alkali, etc.; resistance to thermal deformation; and freeness from foreign materials on the surface and interior. However, there has arisen in recent years, a demand for base films for photography and photoengraving of higher quality, that is, films having excellent transparency and excellent slipperiness, which are contradictory properties, have come to be demanded.
In order to improve slipperiness of polyester films, it has been attempted to form unevenness on the surface of the film by deposition of minute inert particles of catalyst residue or by the addition of minute inert particles of inorganic compounds. As long as the conventional film-making conditions are followed, however, there has been a problem that as the roughness of the surface is increased by formation of unevenness in the surface, the haziness of the surface is increased, and voids between the particles and the polymer are increased and thus the internal haziness is also increased and the overall transparency is abated.
We previously found that an improved film having good transparency and slipperiness was obtained by specifying the degree of planar orientation in the film surface (.DELTA.P) and the average refractive index (n) of the film and forming unevenness units each comprising a protrusion (particle) and a recess with a longer diameter of 3 .mu.m around the protrusion (Japanese Patent Application No. 59-227785).
In order to obtain such a film, it was necessary to longitudinally stretch the film so that the birefringence (.DELTA.n) after longitudinal stretching is very low. For that purpose, hard chromium-finished mirror rolls were used in order to eliminate scratching and stains due to sticking. This required an extremely low longitudinal stretching ratio and tended to contain productivity. If the film was stretched at higher temperature in order to enhance the ratio of longitudinal stretching, it was revealed stains due to sticking and scratches increased. In order to attain low .DELTA.n after longitudinal stretching, it was necessary to longitudinally stretch in two or more steps so as to attain uniformity in thickness of the film, especially flatness of the film after biaxial stretching and heat-setting. Usually, thicker films having good transparency and slipperiness are manufactured by single step longitudinal stretching. If two-step stretching is required, the installment of new equipment is required, which invites increase in manufacturing cost. Control of the film-making in the two-step stretching is more difficult than in the case of one-step stretching. In the case of the manufacture of the film of this invention, in which defects like scratch must be extremely avoided, it was revealed that stretching in two or more steps is not suitable.
Thus to obtain transparent slippery film having excellent flatness, there was a need to develop a new process which provides such films with one step stretching using hard-chromium finished mirror rolls.
In consideration of the above-described problems, we found on the basis of intensive studies that a film having uniform thickness, good flatness, transparency and slipperiness can be obtained by one step stretching only if the film satisfies the following formulas (1) to (7) after being biaxially stretched and heat-set.